He [i.e. Hugh] related also that not many years before a certain John, a king and priest who dwells beyond Persia and Armenia in the uttermost East and, with all his people, is a Christian but a Nestorian, made war on the brother kings of Persians and Medes, called Samiardi, and stormed Ekbatana (the seat of their kingdom).

When the aforesaid kings met him with an army composed of Persians, Medes and Assyrians a battle ensued which lasted for three days, since both parties were willing to die rather than turn in flight. Prester John, for so they are accustomed to call him... emerged victorious.

He said that after this victory the aforesaid John moved his army to the aid of the Church in Jerusalem. But that when he had reached the river Tigris and was unable to transport his army across that river by any evidence he turned towards the north... tarried there for several years... [and] was forced to return home.” 

In analyzing the anecdote that arguably sparked the Prester John fever across Europe, Niayesh (p. 157) notes the structural "ambivalence" of Hugh's account, noting that his story was "caught half-way between the pagan past of classical authorities and the present of Christian Crusaders" insofar as Prester John is "made to fight the long extinct nations of the Medes and Assyrians, rather than directly facing contemporary 'Saracens.'

Even the somewhat contemporaneous historical details do not, in actuality, herald a Christian savior of western Europe. Although this rumor spawned the centuries-long belief in an Eastern potentate capable of uniting Christendom, the initial account of an Eastern anti-Islamic leader was later understood to refer to the deeds of the Qara Khitai, a nomadic Chinese tribe descending from Manchuria. Significantly, this battle took place in Samarkand, not Ecbatana, as Hugh reports. 

Nevertheless, despite historical mistranslation and Iohannes's failure to reach even Byzantium, this rumor helped set in motion, for many Europeans, a belated recognition of the world beyond the Tigris.

Brewer edits and translates the relevant passages of the chronicle (pp. 43-45).

"Of interest to the discussion of the legend of Prester John is a passage in which Benjamin described a powerful king in the East. According to the narrative, the king’s name was Kofar-al-Turak, and this Asian king successfully destroyed the king of Persia. Benjamin claimed that Kofar-al-Turak’s forces 'slew many of the Persian army, and the king of Persia fled with only a few followers to his own country.' The account seems contemporaneous with the 1141 defeat by the Kara-Khitai of the Kara-Khanids, who were nominally vassals of the Seljuks. The idea that the forces of Islam could be defeated by conquerors from the East – especially if they were fellow Christians – no doubt was welcome news in Europe. The series of twelfth and thirteenth century papal and royal embassies to the Turkic nomads known collectively as the Mongols was in part due to the credence placed in the account of Benjamin of Tudela." 

Makhtbhanuth Zabhne (c. 1258-1286)

Bar-Hebraeus's Chronicle, written in Syriac, aspires to narrate world history from Creation until the current day, in two books (concerning secular and sacred history, respectively).

In the Chronicon Syriacum, which concerns civil and political history, Bar-Hebraeus records an occurrence in the early eleventh century that connects to the legend of Prester John. According to the Chronicon the Mongol Keraits of the East Steppe adopted Nestorian Christianity in 1007. This is significant insofar as the early European legends speaking of a powerful Indian prince John cast this figure too as Nestorian and it was not known at that time whether or not Nestorian Christianity had spread that far east. 

Bar-Hebraeus then goes on to mention that it was a King David who, as chief of these same Keraits, was defeated in 1202 by Genghis Khan, who was once King John's vassal. The Syriac documentation of this events matches those of western authors, including the narrative of Marco Polo's journey. There, however, King David/Prester John is known as Ong Khan.  

From Silverberg:

How are we to account for William’s [of Rubruck] linking of Togrul and Kuchluk the Naiman (‘King or Presbyter John’)? They were in fact not brothers, nor of the same tribe, nor of the same generation, nor could Togril, who died in 1203, possibly have succeeded to the throne of Kuchluk, who outlived him by sixteen years. A clue to the source of his error can be found in the Chronicon Syriacum of the Syrian cleric Gregory Abulfaraj Bar-Hebraeus, who lived from 1226 to 1286: speaking of the conversion of the Keraits to Christianity, he notes that in the time of the Mongol dominion they were ruled by an ‘Ung Khan who is called Malik Yuhanna,’ that is, ‘King John.’

Itinerarium fratris Willielmi de Rubruquis de ordine fratrum Minorum, Galli, Anno gratia 1253 ad partes Orientales (c. 1253-1255)

William of Rubruck's Itinerarium records the Franciscan missionary's quest, ordered by King Louis IX, to convert the Tatars to Christianity and to gauge their potential as crusading allies.

Like John of Plano Carpini's Ystoria Mongalorum, William's Itinerarium records basic observations about the geography, culture, religion, and daily life of the Mongols. William's account has been more historically useful due to his incisive observations and his clear writing style. As Brewer (p. 162) notes, William was the first European to cast doubt on the veracity of the Prester John legends, though he does buy into Prester John's historical reality.

In Chapter 17 William writes of a powerful Naiman (Nestorian) shepherd who rose to power as King John at the beginning of the twelfth century during the power vacuum created by the death of the "Gürkhan." William is careful to note that the rumors that began to spread about this King John are exaggerations.   

From Dawson’s English translation (122-23):

In a certain plain among these pasture lands [of the Carcathayans] was a Nestorian, a mighty shepherd and lord of all the people called Naimans, who were Nestorian Christians. On the death of Coir Chan, this Nestorian set himself as king and the Nestorians called him King John, and they used to tell of him ten times more than the truth. For the Nestorians coming from these parts do this kind of thing-- out of nothing they make a great rumour… I passed through his pasture lands and nobody knew anything about him with the exception of a few Nestorians.

This John had a brother by the name of Unc, like him a mighty shepard, and he was beyond the pastures of these Carcathayans, separated from his brother by a distance of three weeks’ journey, and he was lord of a little town called Caracorum,... King John died without an heir and his brother Unc grew rich and had himself called Chan and used to send his herds and flocks as far as the Mongol boundaries… And the Tartars and Mongols made him their leader and captain.” 

More on William of Rubruck.

But now in the city of Acre, I... await the arrival of pilgrims with great longing. Indeed, I believe that if we had 4,000 men of arms, through God's favour we would not be able to find anyone strong enough to resist us. Indeed, there is a great discord amongst the Saracens, and many of them, knowing their error for certain, if they dared to and had the help of Christians, they would be converted to the Lord. I also believe that the Christians living amongst the Saracens are greater in number than the Saracens [themselves]. Also, many Christian kings living in the Easter regions up to the land of Prester John, hearing of the arrival of the crusaders, would come to their help and go to war with the Saracens.

 

Later in the letter, Jacques explains that these "Prester John Christians" are Jacobites, or monophysites​, an interesting contrast to the general trend of identify Prester John's people as Nestorians, who practice a dyophysite belief about Christ. 

A few years later, Jacques expands on this faith by integrating the figure of King David, borrowed from a report called the Relatio de Davide that Jacques assimilated into the framework of the Prester John legend, as demonstrated in this so-called "Letter VII."

In 1876 the Russian scholar Philipp Bruun published a work entitled The Migrations of Prester John, in which he challenged the whole notion that Bishop Hugh's story was a distorted version of the exploits of Yeh·lu Ta-shih. According to Bruun, the prototype of Prester John was the general Ivané (John) Orhelian, commander-in-chief of the army of the kingdom of Georgia. This John Orhelian is One of his f country's national heroes, who fought valiantly for many years to drive the Turks from the Caucasus. In 1123-24 he recaptured from the Seljuks a wide strip of territory in eastern Georgia, including the cities of Tiflis and Ani, and his grateful monarch, King David the Restorer, bestowed on him large grants of land in the reconquered region.

Bruun raised the interesting point that Otto of Freising apparently confused the Georgian city of Ani with the old Persian city of Ecbatana. In a passage of Otto's chronicle somewhat earlier than the Prester John anecdote, Otto, in providing some geographical information apparently received from Bishop Hugh, remarked, "The kings of the Persians ... . have themselves established the seat of their kingdom at Ecbatana, which ... in their tongue is called Rani." The defeat of the Seljuks at Ani in 1123 thus begins to seem a more plausible source for Prester John's victory at Ecbatana than does the triumph of Yeh-lü Ta-shih outside Samarkand.  Moreover, John Orbelian was a Christian-- Greek Orthodox, though, and not Nestorian. And, though he was neither a king nor a priest, the Georgian general did conduct himself in regal fashion: he dined on silver dishes, had the privilege of sitting on a couch at royal banquets while the other princes sat merely on cushions, and the Orbelian family held the hereditary right to provide over the cornations of Georgian kings.